Gear pumps are popular for precise transfer and metering applications such as automatic beverage dispensers. A typical gear pump includes a pump housing defining a chamber, two gears received in the chamber, and a liquid inlet and a liquid outlet which are in fluid communication with the chamber. One of the two gears is a driving gear driven by a motor and the other one is an idle gear driven by the driving gear. The two gears rotate against each other so as to create the pump action. Liquid entering the pump inlet is drawn into the space between the inner surface of the chamber and teeth of the rotating gears and then is forced through the pump outlet under pressure. The flow rate of the liquid can be accurately controlled by the rotational speed of the gears, which can be regulated by varying the speed of the motor.
In some beverage dispensers the pumping of hot liquid is required. As the gears of the gear pump are immersed in the hot liquid, the gears expand creating excessive engagement between the two gears leading to excessive wear or jamming of the gears, neither of which is desired. This can be avoided by appropriate sizing of the gears to allow for the expansion, however this means that the pump does not operate efficiently with cold or warm liquids. Also, as the gear pump is in direct contact with a liquid for human consumption, the pump must satisfy the requirements of certain food safety standards.